1. Field of the Invention
The present invention relates to electron beam testers, and particularly to electron beam testers used in probing of integrated circuits.
2. The Prior Art
Electron-beam test systems for probing, scanning and diagnosing VLSI (very large scale integrated) circuit devices are known in the art. See, for example, U.S. Pat. No. 4,706,019 to Neil Richardson, which is incorporated herein by this reference. FIG. 1 is a schematic block view of such a system 110, comprising a test probe 112, a circuit exerciser 114, a data processing system (DPS) 116, and a display terminal 118. Test probe 112 and circuit exerciser 114 are controlled by data processing system 116 in response to commands entered by an operator, while a circuit device under test (DUT) mounted within test probe 112 is exercised by signals from exerciser 114. Electron-beam test systems of this type are commercially available, such as the IDS 5000 system offered by the Diagnostic Systems Division of Schlumberger Technologies, Inc., San Jose, Calif.
FIG. 2 is a functional block diagram of portions of a typical prior-art system of the kind shown in FIG. 1, as configured for voltage measurement. Circuit exerciser 114 applies a repetitive pattern of signals to exercise the DUT, and sends timing signals to a timebase circuit 210 within test probe 112. Timebase circuit 210 controls a beam pulser 215 which supplies a train of control pulses to a beam blanker 220. In response to the control pulses from beam pulser 215, beam blanker 220 periodically unblanks a beam of electrons produced by an electron source 225. The resulting train of electron-beam pulses 235 is finely-focused in an electron optical column 230. Electron-beam pulses 235 are directed at a conductor of interest inside the DUT.
Voltage measurement is achieved by detecting and analyzing the energies of secondary electrons 240 which are generated during interaction of the electron-beam pulses 235 with the DUT. An electron detector 245 comprising a scintillator 247 and a photomultiplier 249 detects the secondary electrons 240, and provides a detector output signal. The detector output signal is passed through a buffer 250 to a gate switch 255. The closing of gate switch 255 is controlled by signals from a gate pulser 260. Gate pulser 260 is in turn responsive to control signals from timebase 210 which are synchronized with the control signals to beam pulser 215. Closing of gate switch 255 causes the buffered detector output signal to be sampled. The samples are supplied to an integrator circuit 260.
The measured voltage output from integrator circuit 260 is added to an offset voltage V.sub.Offset in a summer 265, and the result is passed via a controllable switch 270 to a buffer 275. A filter voltage V.sub.Filter is supplied by buffer 275 to a secondary-electron retarding grid 280. Offset voltage V.sub.Offset sets the operating point of retarding grid 280. Switch 270 is operated by a scan generator 285 which also controls the beam deflection coils (not illustrated) within electron-optical column 230. During waveform (voltage) measurement, scan generator 285 produces a waveform-mode signal which maintains switch 270 in the "W" position (as illustrated), and supplies a beam-vectoring signal to the beam deflection coils to direct the electron-beam at a conductor of the DUT to be probed.
Samples may be taken once for each repetition of the test pattern applied to the DUT by circuit exerciser 114, although typically the samples are taken at multiple sampling intervals during the test pattern repetition. Integrator circuit 260 integrates the samples from a number of repetitions of the test pattern applied to the DUT to produce a voltage measurement signal at its output. It is noted that the integrator circuit 260 is shown only schematically for simplicity of illustration. If samples are taken during multiple sampling intervals of the test pattern, the samples are integrated interval-by-interval. That is, the samples from the first interval of multiple repetitions of the test pattern are integrated to produce a voltage measurement for the first interval, the samples from the second interval of multiple repetitions of the test pattern are integrated to produce a voltage measurement for the second interval, and so on. The voltage measurements for the multiple sampling intervals are typically displayed and/or recorded as a measured waveform.
High-bandwidth measurements within the DUT can thus be made by using the pulsed electron beam as a sampling probe, much like a sampling oscilloscope. The measurement bandwidth can be significantly greater than the bandwidth of the electron detector, so that the bandwidth of the electron detector is of little or no importance in determining the overall measurement bandwidth.
The measured waveform is determined by the convolution of the sampling pulse shape and the actual waveform on the conductor of the DUT being probed, as will now be explained with reference to FIG. 3. Line 310 shows an ideal electron-beam sampling pulse 312 having zero amplitude except for an infinitesimal duration at which it has a fixed, non-zero amplitude. In other words, the ideal sampling pulse is the well-known Dirac delta function, in which .delta.(t)=0 for t.noteq.0; .delta.(t).noteq.0 for t=0; and ##EQU1##
In practice, ideal sampling is not achievable. As shown in line 320 of FIG. 3, a practical sampling pulse 322 has a finite duration .DELTA.t, which limits the measurement rise-time. Also, the sampling pulse signal never truly achieves a zero amplitude. Forward scattering of electrons from the beam blanker (beam leakage) causes the sampling signal to have a background amplitude A as shown in line 320. These forward-scattered electrons find their way to the DUT and perform an unwanted, low-level sampling function. When the secondary-electron signal from electron detector 245 (FIG. 2) is sampled by sampling gate 255 (FIG. 2) as represented by the gate function 332 at line 330 of FIG. 3, the resultant sampling function 342 is as shown in line 340 of FIG. 3. It will be seen in line 340 that the resultant sampling function 342 includes not only the beam pulse of interest, but also an unwanted component of background noise of level A.sub.1 due in large part to interaction of forward-scattered electrons with the DUT.
The source of the unwanted, forward-scattered (beam leakage) is explained with reference to FIG. 4. In conventional electron-beam testers, the beam is blanked using some kind of beam deflection apparatus and a beam limiting aperture before and after the deflection apparatus. See, for example, U.S. Pat. No. 4,721,909 to Neil Richardson, which is incorporated herein by this reference. Electrons from source 225 are directed toward a first beam-limiting aperture 410. The portion of the electron-beam which passes through aperture 410 is, if not deflected away from aperture 420 as illustrated in FIG. 4, directed at a second aperture 420 which serves as an optical entrance pupil to electron optical column 230 of FIG. 2. Deflection of the electron beam away from aperture 420 as illustrated in FIG. 4 is accomplished by charging deflection plates 425 and 430 with respective deflection voltages-V and -V. However, even when the electron beam is supposed to be blanked, there is always a small amount of forward scattering of electrons (shown at 435) from the first aperture 410.
The typical signal processing electronics shown in FIG. 2 employs a "boxcar integrator" (e.g., gate 255 and integrator 260) which gates the electron-detector signal to remove the contribution from scattered electrons except during the "gate-open" time. The "gate-open" time is the interval during which gate switch 255 is closed to allow the electron-detector signal to pass to integrator 260. The gate-open time can be kept to a few nanoseconds if a very high bandwidth electron detector is used and if the secondary electrons are accelerated after they are emitted from the DUT so that transit-time spread through the electron detector is minimized. However, it is often preferred to use a more robust electron detector such as a scintillator, which has a lower bandwidth and a response time of about 100 nanoseconds. The scintillator's slower response time necessitates a gate-open time on the order of a few hundreds of nanoseconds. In this case, the contribution of the forward-scattered electrons to the detector-signal sample is not negligible.
For example, if the nominal sampling pulse is 100 picoseconds, the gate-open time (gate width) is 300 nanoseconds, and the beam extinction factor is 1/3000 (e.g., pulse width =1 nanosecond, gate width =300 nanoseconds), there will be equal contribution from the forward-scattered electrons and the nominal sampling pulse. This gives rise to errors in the measured waveform which can be misleading to an unsuspecting operator, as illustrated in lines 510-530 of FIGS. 5. Line 510 shows an ideal step function to be measured, having a step at 512 of 5.0 volts.
Line 520 shows a practical sampling function 522 in which the unwanted contribution 524 from forward-scattered electrons (from level 0 to level A) is about equal to the contribution 526 from the nominal sampling pulse. Convolving the sampling function of line 520 with the step function of line 510 produces a resulting measured waveform 532 as shown in line 530. Ideally, one would require that the extinction ratio be nearer 1/300,000, but this is well beyond what is typically achieved.